Thin film electroluminescent (TFEL) devices are utilised primarily to provide the light source for flat panel emissive displays, see, for example reference (1) and references therein. A typical TFEL display structure is shown in schematic cross-section in FIG. 1, where a light emitting phosphor thin film is sandwiched between dielectric cladding layers to form a capacitative structure. Electrodes are disposed on the outer surfaces of the cladding dielectrics, such that the application of a high voltage ac signal between these electrodes results in the emission of light form the phosphor thin film, due to the high electric field strength in the phosphor film being greater than a threshold value. For typical display applications, the thin films are deposited onto a transparent substrate, wish one of the electrode layers also being transparent, such at light generated within the phosphor layer may be viewed directly. This is termed surface emission.
A significant disadvantage associated with conventional surface emission is the fact that up to 90% of the light generated by thin film phosphors is tripped within the phosphor layer by virtue of internal reflection, and is transmitted laterally, as shown in FIG. 2. This occurs because phosphor thin films generally have a refractive index that is higher than that of the materials used to clad them. This effect was first reported by D H Smith in 1982, J Lum 23 (1983) 209 when he observed that the light intensity emitted at the edge of a TFEL thin film display was much greater than that emitted directly though the surface. Recognising this as a loss mechanism, several groups have attempted to improve the luminous surface emission by reducing the internal reflection, so as to allow more light to be emitted directly through the thin film surface. Described, for example in U.S. Pat. No. 5,131,065 (Boeing) is a method of improving the surface emission from TFEL panels by index matching the optical properties of the phosphor and dielectric layers, such that internal reflection is minimised. Alternatively, U.S. Pat. No. 5,072,152 (Planar Systems), and U.S. Pat. No. 4,774,435 (GTE Labs) both teach the use of rough non-planar interfaces between the phosphor thin film and the cladding layers. The Planar Method is to initially deposit a dielectric layer onto a planar substrate, where the dielectric is deposited by electron-beam evaporation in order to generate a rough surface contour. This "convoluted" surface contour is then replicated by the thin films deposited on top, such that the internal light reflection is reduced and more light output is provided at the front of the panel. The GTE method is to produce the same effect--a rough, non-planar surface at the phosphor/dielectric interface, but in this case it is achieved by depositing the thin films onto a substrate that has a rough non-planar surface. In each case, the technique used is intended to overcome the inherent internal reflection by increasing the probability that the angle of incidence for a light ray arriving at the phosphor/dielectric interface will be less that the critical angle and will thus be transmitted.
The methods described above, if successfully implemented, will undoubtedly improve the direct surface emission through the various thin films. However, in doing so they effectively eradicate a property of TFEL devices that may in fact be highly beneficial; i.e. the confinement of up to 90% of the generated light to within the geometrical limits of the phosphor thin film. Since such phosphor thin films are in general of the order of 1 micron thick (10.sup.-6 m), the result is the concentration of the light energy to a microscopic area. This consequently increases the intensity relative to that of conventional surface emission, where approximately 10% of the light energy is spread over an area of typically 100 micron.sup.2.
The gain in emitted light intensity (luminance) that may be attained via the utilisation of laterally transmitted light was demonstrated by D H Smith in 1982, J Lum 23 (1983) 209 when he observed that the luminous efficiency of a TFEL display could be increased by including the light that was unintentionally emitted from the edge of the thin film stack. This concept was then further developed by workers from Westinghouse Corp and Edge Emission Incorporated into methods of producing high intensity light sources for image bar arrays intended to work in electrophotographic printers. Described in US and European patents: U.S. Pat. No. 4,535,341, U.S. Pat. No. 4,685,448 and EP 0 398 591 A2 are devices where the light is emitted in a direction parallel to the substrate, directly from an exposed edge of the phosphor film. This is similar, in principle, to the structure of an edge emitting LED, or diode laser, where spatially confined light is channelled to an emitting facet, which is typically formed by cleaving the substrate as described by J. Wilson, and J. F. B. Hawkes; in OptoElectronics An Introduction, Prentice Hall International, (1983) 211. However, as detailed in U.S. Pat. No. 4,535,341, the TFEL edge emitting linear arrays are formed by defining an exposed emitting facet in the TFEL stack directly at the edge of the substrate. In such a configuration, the proximity of both the emitting facet, and the electrodes to the substrate edge is problematic due to the high voltages required to operate TFEL devices and due to the sensitivity of the EL materials to the effects of exposure to moisture and other contaminants. Attempts to overcome these problems are consequently concerned with protection of the emitting edge, and the isolation of the electrodes across which the high voltage drive signal is applied. For example, in U.S. Pat. No. 4,734,723, an optical printer head is disclosed that is formed from a plurality of edge emitting TFEL devices positioned along one edge of the substrate, with waveguide strips to transmit the light from the EL device to the other end of the substrate, similar to the operation of an edge emitting LED. While protecting the TFEL edge emitter, this presents additional optical attenuation, and may thus reduce the benefits of edge emission in regard to the intensity of light produced. Alternatively, as disclosed in EP 0 398 591 A2, a practical edge emitter light source requires an integral housing, or packaging assembly to protect the device. Such an assembly is described, which consists of a series of spacers and packaging members arranged around the TFEL substrate and sealed to prevent contamination. Again, the extra optical interfaces that result from such packaging may be detrimental to the emitted light intensity, and the packaging process itself, requiring precision mechanical alignment, is costly and difficult.